Providing a Matcap fixes it, but I don't need one for this effect.
Shader error in 'd4rkpl4y3r/Optimizer/M_Hair Poiyomi Toon': undeclared identifier '_MatcapPan' at Packages/d4rkpl4y3r.d4rkavataroptimizer/TrashBin/ZZZ3def52bcba776384b1f68b2b95d67a5a.cginc(3717) (on d3d11)
I talked to poi about it and it will be fixed in a future poi version.
It also causes the lock in to be pink when having no matcap texture but the pan is marked as animated.
Tested on Poiyomi 9.0.38 and 9.0.40
Providing a Matcap fixes it, but I don't need one for this effect.